Elastomeric proteins, and in particular elastin, have been the subject of extensive investigations aimed at a molecular understanding of the structure-function relationship among these proteins, their mechanical properties, environmental sensitivity and self-assembly properties. These studies indicated a role for disordered protein structures in proteins and protein domains, and the identification of short recurrent peptides which were capable of forming protein-polymers with similar structural and environmental properties. A general elastic, environmentally responsive motif Val-Pro-Gly-X-Gly (SEQ ID NO: 135) was found in elastin, where X is any amino acid except Proline, and was used to develop elastin-like polypeptides (ELPs) for biotechnological and biomedical uses. Similarly, resilin-like polypeptides displaying mechanical properties comparable to the native resilin protein were made.
Silks, on the other hand, constitute a complex family of proteins that encompasses both elastomeric and non-elastomeric proteins. Elastomeric silks include a highly repetitive GPGGX (SEQ ID NO: 1) motif. Studies have indicated that there is an absence of Proline residues in non-elastomeric silks, and that β-sheet structures increase in proportion to the GPGGX (SEQ ID NO: 1) content. However, the structures adopted by the abundant GPGGX (SEQ ID NO: 1)/GPGQQ (SEQ ID NO: 2) and GGX repeat units remain unclear. Little progress has been made on the design of recombinant silk-like biomaterials. In addition, elastin remains the only elastomeric repetitive protein successfully reduced to a short motif capable of displaying both elasticity and environmental sensitivity.
After more than three decades of research since the discovery of the environmental sensitivity of elastin monomers (tropoelastin), the more than two decades since the identification of the canonical elastomeric motif VPGXG (SEQ ID NO: 3), and the almost two decades since the generalization of this repeat unit into the canonical ELP motif VPGXG (SEQ ID NO: 3), only a handful of elastin-inspired polypeptides departing from the canonical sequence have been uncovered, namely minor modifications of the canonical motif, such as LPGXG (SEQ ID NO: 4) and IPGXG (SEQ ID NO: 5), and the repeat unit VPAVG (SEQ ID NO: 6). Recent efforts have made use of complex bioinformatics tools to search for sequence conservation, amino acid patterns, and recurrent motifs among elastin proteins from different species; these studies have hinted at the potential functional role of the PG dipeptide in elastin (see, e.g, He, D. et al. (2007) Matrix Biology 26:524-540). In a more general approach, studies of similarities in Proline and Glycine content between a large panel of elastomeric proteins from different species including elastin, resilin, gluten, and silks failed to identify first principles for the design of general elastomeric motifs, or to identify and reduce to practice novel motifs responsible for the elasticity and/or environmental sensitivity of these proteins. (see, e.g., Rauscher, S. et al. (2006) Structure 14:1667-1676).